Light-emitting diodes (LEDs) are a commonly used light source in applications including lighting, signaling, signage, and displays. LEDs have several advantages over incandescent and fluorescent lamps, including high reliability, long lifetime, and high efficiency.
A typical prior art LED system is shown in FIG. 1. An LED is placed on a circuit board, which carries electrical current to and from the LED, and allows heat generated by the LED to flow from the relatively small LED to a larger area, from which the heat then escapes to air. Often a heat sink is added to facilitate the heat flow to air. Light is emitted by the LED in a generally upward direction, with a wide (typically ±60–90°) angle. Many applications, such as f narrower, more collimated emission angle. For this purpose secondary collection optics are usually added to redirect the light rays more narrowly upward. (These optics are called “secondary” to distinguish them from the “primary optics” of the LED itself, such as the dome shown in FIG. 1.)
These secondary optics are shown schematically in FIG. 1. The schematic shows the most common configuration, in which light enters one side of the collection optics at a wide angle, and exits a second side at a narrower angle. Typical secondary optics include lenses and reflectors. The optics can be lenses, Fresnel lenses, parabolic reflectors, conical reflectors, compound parabolic concentrators, light pipes, or many other shapes. FIG. 2 shows example arrays of each, taken from “Application Note 1149-5, Secondary Optics Design Considerations for Super Flux LEDs,” by Lumileds, Inc. Note that, regardless of the specific design, the optics are most commonly positioned in between the LED and the desired direction of narrow-beam light emission, with the circuit board and heat sink on the opposite side of the LED.
An alternative approach is to use a mirror to reflect light back in the direction of the light source, as shown in FIG. 3. A large variety of reflector shapes are known using this approach, including paraboloidal, ellipsoidal, spherical, compound conic, and faceted. It is well-known in the art of optical design that this approach has significant advantages. Very wide angles can be collected and collimated, and the overall system can be quite compact. For example, for a light source emitting into ±90°, like an LED, a paraboloidal reflector will have a height-to-diameter less than or equal to 0.5. For the approach shown in FIG. 2, however, the height-to-diameter ratio is typically larger, often 1.0 or more. On the other hand, one disadvantage of the approach shown in FIG. 3 is that the light source blocks a portion of the outgoing light. For very narrow angles (<±10°), and if the physical structure supporting the light source is not much larger than the light source itself, then the reflector is typically much larger than the blocking area and the blocked fraction is minimal (<3–5%).
The back-reflector approach of FIG. 3 has been relatively little used with LEDs, however. One major problem is that the circuit board and heat sink are typically much larger than the light source itself. If the heat sink is positioned in the beam path close to the LED, then an unacceptably high fraction of light would be blocked. If the heat sink and circuit board are remotely positioned on the periphery of the beam, then the thermal path to the heat sink is undesirably long and the thermal resistance (temperature increase divided by power applied to the LED, units of ° C./W) is undesirably high.